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Mysteries of Venus revealed at wavelengths invisible to human eyes

05.12.2008

New images taken by instruments on board ESA’s Venus Express provide a unique insight into the windy atmosphere of our neighbouring planet and reveal that global patterns at the Venus cloud tops are the result of variable temperatures and cloud heights.

Using the spacecraft’s ultraviolet and infrared cameras, the Venus Express team, including UK scientists, have been able to compare what the planet looks like at different wavelengths, allowing them to study the physical conditions and dynamics of the planet’s atmosphere. These results appear today (4th December) in the journal Nature.

Professor Fred Taylor, one of the Venus Express scientists, from Oxford University and funded by the UK’s Science and Technology Facilities Council, said, "The features seen on Venus in ultraviolet light have been a puzzle to astronomers for nearly a century. These new images have revealed the structure in the clouds that produces them and shows how they result from complex meteorological behaviour. We can now study in much greater detail and try to understand the origin of features such as the large hurricane-like vortex over the north and south poles. Like many things on Venus, including global warming, this feature has similarities to atmospheric and environmental process on Earth, but the Venus version is much more extreme.”

Observations made with the ultraviolet camera show numerous high-contrast features. The cause is the uneven distribution of a mysterious chemical in the atmosphere that absorbs ultraviolet light, creating bright and dark zones. But the exact chemical species that creates the high-contrast zones still remains elusive. Most simple candidates have been ruled out, and a complex compound of sulphur is now the favourite. It will probably take measurements inside the clouds to identify it, but we do know that Venus’ atmosphere is loaded with sulphur from volcanic eruptions on the surface below.

The ultraviolet images also reveal the structure of the clouds and the dynamical conditions in the atmosphere, whereas the infrared data provides information on the temperature and altitude of the cloud tops.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, said “These new images provide us with a wealth of information about the atmospheric conditions of this fascinating neighbour planet. We can now study Venus in greater detail to understand more about its complex processes. “

With data from Venus Express, scientists have learnt that the equatorial areas on Venus that appear dark in ultraviolet light are regions of relatively high temperature, where intense convection brings up the mysterious dark material from below. In contrast, the bright regions at mid-latitudes are areas where the temperature in the atmosphere decreases with depth, which prevents air from rising. The effect is most extreme in a wide belt around each pole, nicknamed the ‘cold collar’, which appears darkest, hence coldest, in infrared measurements, but as a bright band in the ultraviolet images.

Observations in the infrared have been used to map the altitude of the cloud tops. Surprisingly, the clouds in both the dark tropics and the bright mid-latitudes are located at about the same height of about 72 km above the surface. At 60 degrees of latitude, the cloud tops start to sink, reaching a minimum of about 64 km at the ‘eye’ of a huge hurricane-like vortex at the pole, which measures about 2000 km across and rotates around the pole in about 2.5 days.

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